The present invention is in the field of video TV imaging reduction and in particular concerns ghost cancellation application.
For a better understanding of the background of the invention, there follows a brief description of the so-called ghost phenomenon in TV video imaging. Ghosts are created by reflections of the broadcast TV signal from large objects such as mountains and buildings. The reflections of the main signal appear at the TV antenna as delayed, or preceded and attenuated duplicates of the main signal. When the TV antenna receives the reflections together with the main broadcast signal, they are displayed on the screen as xe2x80x9cghostsxe2x80x9d.
The interference caused by ghosts not only annoys the viewer, but also degrades the correct operation of the TV circuits. To facilitate the operation of the TV, synchronization signals are transmitted together with the video signal. Reflections that cause ghosts not only distort the image, but also distort the teletext signal.
In accordance with prior art technique, the interference of strong ghosts cannot be avoided or corrected, unless the TV device has a so called ghost canceller module and the broadcast TV signal has GCR (ghost canceling reference) incorporate therein. The Ghost Canceller extracts the GCR (Ghost Canceling Reference) out of the received video, if it is broadcast, and compares it (by means of subtraction) to the expected GCR tat is a-priori stored in the application memory. If the subtraction is approximately zero, this indicates that no ghost (or other interference) is induced, otherwise an error is produced which indicates that ghost exists. In the latter case, a correlation is applied between the error and the input (output in modified LMS algorithm, used by the Ghost Canceller), set of filter taps is built, for compensating the ghost phenomenon and bring about ghost-free image.
A GCR whilst giving rise to the ghost cancellation is not always present in the TV broadcast signal.
There is accordingly a need in the art to provide for a system and method which enables ghost reduction (referred to also as xe2x80x9cdeghostingxe2x80x9d) from TV image (referred to generally also as composite video signal), whilst avoiding the use of GCR.
In accordance with the a broad aspect of the invention the ghost constituent of a composite video signal is reduced without using a GCR, but rather processing at least one teletext data line constituent (referred to occasionally, in short, also as teletext line) of the composite video signal so as to obtain a ghost induced error. The ghost induced error is then utilized to calculate filter coefficients that are fed to a filter. The filter having the specified filter coefficients is applied to the input composite video signal (after handling errors that stem from asynchroneouous sampling, as will be explained in detail below) for equalization step giving rise to substantially deghosted composite video signal, i.e. a video signal where the ghost constituent is reduced or even eliminated.
Accordingly, in accordance with the broadest aspect of the invention there is provided a method for reducing a ghost constituent of a composite video signal; the composite video signal includes a tele-text data lines constituent; the method comprising the steps of:
(a) detecting and processing at least one teletext data line constituent so as to obtain an ghost induced error; and
(b) utilizing said ghost induced error for calculating filter coefficients.
(c) applying said composite video signal to a filter having said filter coefficient for obtaining substantially deghosted composite video signal.
There is further provided a system for reducing a ghost constituent of a composite video signal; the composite video signal includes a teletext data lines constituent; the system comprising:
(a) a unit for detecting and processing at least one teletext data line constituent so as to obtain an ghost induced error; and
(b) a unit utilizing said ghost induced error for calculating filter coefficients
(c) applying said composite video signal to a unit having said filter coefficient for obtaining substantially deghosted composite video signal.
In accordance with a preferred embodiment, the deghosting process is improved by enhancing the high frequencies of each teletext line used for the subsequent equalization process. Having done this, the base filter on which the equalizer builds its taps in an iterative manner (as will be explained in detail below), is a lowpass filter, therefore it helps he equalizer to accomplish enhanced performance in a relatively short time.
The pre-processing of teletext data lines (before running the equalization process that brings about the deghosted signal is required in order to compensate for the off-phase sampling due to the asynchronous sampling of the teletext data line (or lines). The compensation will bring about synchronized sampling with the teletext data lines. By a specific embodiment, this is accomplished by determining the sampling phase error by filtering the teletext synchronization pattern with matched filter, and then by approximate the peak of the matched filter result to a 2nd order (parabolic) polynom.
By a specific embodiment, the step of determining the sampling phase error utilizes the Viehta rule for the peak position. By this embodiment the specified compensation for the off-phase samples includes calculating filter coefficients hI which will enable to interpolate the in-phase samples so as to achieve interpolated synchronized sampling with the pre-defined teletext data line samples.